1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor thin film, and particularly to method of manufacturing a thin film transistor (TFT) using a crystalline semiconductor film containing silicon.
Incidentally, the term “semiconductor device” used in the present specification includes all devices functioning by using a semiconductor, and not only a single element such as a TFT, but also an electro-optical device and application products furnished with the same are also included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique of constituting a semiconductor circuit by forming TFTs on a glass substrate and the like, has been rapidly developed. As such a semiconductor circuit, an electro-optical device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. Furthermore, a system-on-panel having additional built-in logic circuits such as a memory circuit and a clock generation circuit has also been developed.
Since such a driver circuit and a logic circuit are required to be operated at high speed, it is not suitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, under the present circumstances, a TFT using a crystalline silicon film (polysilicon film) as an active layer has become the main stream.
The present inventors disclose a technique for obtaining a crystalline silicon film on the glass substrate in Japanese Patent Application Laid-open No. Hei 7-130652. In the technique disclosed in the publication, a catalyst element for promoting crystallization is added into an amorphous film, and by carrying out a heat treatment, a crystalline silicon film is formed.
This technique can lower the crystallization temperature of the amorphous silicon film by the action of the catalyst element drastically by 50 to 100° C., and the time required for crystallization can also be reduced to ⅕ to {fraction (1/10)}. Further, it is experimentally confirmed that the crystalline silicon film obtained by this technique exhibits excellent crystallinity.
As the above-mentioned catalyst elements, a metal element such as nickel or cobalt is used. Since such a metal element forms a deep level in a silicon film to capture a carrier, there is a fear that the metal element has a bad influence to the electrical characteristics and reliability of a TFT.
Further, the catalyst elements remaining in the active layer of the TFT are confirmed to cause irregular segregation. The present inventors suspected that such a segregated region forms a leak path of weak current, and that it causes an abrupt increase of OFF current (i.e., the current when TFT is in OFF state).
Accordingly, it is preferred that, after the crystallization, the catalyst elements are rapidly removed or reduced to such a low level that they may not influence the electrical characteristics. The present inventors have already filed a patent application in which gettering effect of a halogen element is utilized as a means therefor.
However, since the method proposed above requires a heat treatment at a temperature as high as 800° C. or higher, a glass substrate with a low thermal resistance may not be used. That is, the advantage of employing the low temperature process using catalyst elements cannot be effectively utilized.